319 research outputs found
Gold coating of micromechanical DNA biosensors by pulsed laser deposition
In this work, we describe the gold-coating of silicon microcantilever sensors by pulsed laser deposition (PLD) and their performance as DNA biosensors. To test optimum deposition conditions for coating the sensors, silicon substrates were gold coated by PLD using the fifth harmonic of a Nd:YAG laser (213 nm, pulse duration 15 ns). The gold deposits were characterized by atomic force microscopy and x-ray diffraction. The adequate conditions were selected for coating the sensors with a 20 nm thick gold layer and subsequently functionalized with a self-assembled monolayer of thiolated DNA. To verify PLD as a tool for gold coating of biomechanical sensors, they were characterized by using a scanning laser analyzer platform. Characterization consisted in the measurement of the differential stress of the cantilevers upon hydration forces before and after functionalization with a double-stranded DNA monolayer. The measurements showed that the sensor surface stress induced by the adsorption of water molecules is approximately seven times higher than that of functionalized sensors gold coated by thermal evaporation. These results indicate that gold coating by PLD could be an advantageous method to enhance the response of biomechanical sensors based on gold-thiol chemistry.Peer Reviewe
Differentiation of Human, Dog, and Cat hair Fibers using DART TOFMS and Machine Learning
Hair is found in over 90% of crime scenes and has long been analyzed as trace evidence. However, recent reviews of traditional hair fiber analysis techniques, primarily morphological examination, have cast doubt on its reliability. To address these concerns, this study employed machine learning algorithms, specifically Linear Discriminant Analysis (LDA) and Random Forest, on Direct Analysis in Real Time time-of-flight mass spectra collected from human, cat, and dog hair samples. The objective was to develop a chemistry- and statistics-based classification method for unbiased taxonomic identification of hair. The results of the study showed that LDA and Random Forest were highly effective in separating mass spectra collected from hair samples with accuracies ranging from 94-98%. This approach holds significant promise for forensic investigations, archaeology, and artifact analysis
Mergers of close primordial binaries
We study the production of main sequence mergers of tidally-synchronized
primordial short-period binaries. The principal ingredients of our calculation
are the angular momentum loss rates inferred from the spindown of open cluster
stars and the distribution of binary properties in young open clusters. We
compare our results with the expected number of systems that experience mass
transfer in post-main sequence phases of evolution and compute the
uncertainties in the theoretical predictions. We estimate that main-sequence
mergers can account for the observed number of single blue stragglers in M67.
Applied to the blue straggler population, this implies that such mergers are
responsible for about one quarter of the population of halo blue metal poor
stars, and at least one third of the blue stragglers in open clusters for
systems older than 1 Gyr. The observed trends as a function of age are
consistent with a saturated angular momentum loss rate for rapidly rotating
tidally synchronized systems. The predicted number of blue stragglers from main
sequence mergers alone is comparable to the number observed in globular
clusters, indicating that the net effect of dynamical interactions in dense
stellar environments is to reduce rather than increase the blue straggler
population. A population of subturnoff mergers of order 3-4% of the upper main
sequence population is also predicted for stars older than 4 Gyr, which is
roughly comparable to the small population of highly Li-depleted halo dwarfs.
Other observational tests are discussed.Comment: number of pages depends on font, margins, columns etc (58 with given
format), 14 figures, submitted to the Astrophysical Journa
Probing the LMC age gap at intermediate cluster masses
The LMC has a rich star cluster system spanning a wide range of ages and
masses. One striking feature of the LMC cluster system is the existence of an
age gap between 3-10 Gyrs. But this feature is not as clearly seen among field
stars. Three LMC fields containing relatively poor and sparse clusters whose
integrated colours are consistent with those of intermediate age simple stellar
populations have been imaged in BVI with the Optical Imager (SOI) at the
Southern Telescope for Astrophysical Research (SOAR). A total of 6 clusters, 5
of them with estimated initial masses M < 10^4M_sun, were studied in these
fields. Photometry was performed and Colour-Magnitude Diagrams (CMD) were built
using standard point spread function fitting methods. The faintest stars
measured reach V ~ 23. The CMD was cleaned from field contamination by making
use of the three-dimensional colour and magnitude space available in order to
select stars in excess relative to the field. A statistical CMD comparison
method was developed for this purpose. The subtraction method has proven to be
successful, yielding cleaned CMDs consistent with a simple stellar population.
The intermediate age candidates were found to be the oldest in our sample, with
ages between 1-2 Gyrs. The remaining clusters found in the SOAR/SOI have ages
ranging from 100 to 200 Myrs. Our analysis has conclusively shown that none of
the relatively low-mass clusters studied by us belongs to the LMC age-gap.Comment: 9 pages, 8 figures. Accepted to MNRA
Preliminary conceptual model of the Cerro Blanco caldera-hosted geothermal system (Southern Puna, Argentina): Inferences from geochemical investigations.
The Cerro Blanco Caldera (CBC) is the youngest collapse caldera system in the Southern Central Andes (Southern
Puna, Argentina). The CBC is subsiding with at an average velocity of 0.87 cm/year and hosts an active geothermal
system. A geochemical characterization of emitted fluids was carried out based on the chemical and
isotopic compositions of fumaroles, and thermal and cold springs discharged in this volcanic area with the aim of
constructing the first hydrogeochemical conceptual model and preliminary estimate the geothermal potential.
The main hydrothermal reservoir, likely hosted within the pre-caldera basement rocks, has a Na+-ClË(HCO3)Ë
composition with estimated temperatures â„135 °C. The unconsolidated, fine-grained Cerro Blanco ignimbrite
likely acts as the cap-rock of the hydrothermal system. The presence of phreatic eruption breccias in the surrounding
area of the geothermal fumaroles supports the effectiveness of the pyroclastic deposit as sealing rocks.
The isotopic data of water (ÎŽ18O and ÎŽD) indicate a meteoric recharge of the hydrothermal reservoir, suggesting
as recharge areas the sectors surrounding the CBC, mainly towards the W and NW where large outcrops of the
pre-caldera basement exist. A fault-controlled hydraulic connection between the hot springs and the hydrothermal
reservoir is proposed for the Los Hornitos area. The fumaroles show the typical compositional features of
hydrothermal fluids, being dominated by water vapor with significant concentrations of H2S, CH4 and H2.
Considering the high geothermal gradient of this area (âŒ104 °C/km) and the relatively high fraction of mantle
He (âŒ39%) calculated on the basis of the measured R/Ra values, the hydrothermal aquifer likely receives inputs
of magmatic fluids from the degassing magma chamber. The preliminary geothermal potential at CBC was
evaluated with the Volume Method, calculating up to E = 11.4*1018 J. Both the scarce presence of superficial
thermal manifestations and the occurrence of an efficient cap-rock likely contribute to minimize the loss of
thermal energy from the reservoir. The results here presented constitute the necessary base of knowledge for
further accurate assessment of the geothermal potential and ultimately the implementation of the geothermal
resource as a viable energy alternative for small localities or mining facilities isolated from the National
Interconnected System due to their remote localization.Published1022136A. Geochimica per l'ambiente e geologia medicaJCR Journa
The effect of rotation on the abundances of the chemical elements of the A-type stars in the Praesepe cluster
We study how chemical abundances of late B-, A- and early F-type stars evolve
with time, and we search for correlations between the abundance of chemical
elements and other stellar parameters, such as effective temperature and Vsini.
We have observed a large number of B-, A- and F-type stars belonging to open
clusters of different ages. In this paper we concentrate on the Praesepe
cluster (log t = 8.85), for which we have obtained high resolution, high
signal-to-noise ratio spectra of sixteen normal A- and F-type stars and one Am
star, using the SOPHIE spectrograph of the Observatoire de Haute-Provence. For
all the observed stars, we have derived fundamental parameters and chemical
abundances. In addition, we discuss another eight Am stars belonging to the
same cluster, for which the abundance analysis had been presented in a previous
paper. We find a strong correlation between peculiarity of Am stars and Vsini.
The abundance of the elements underabundant in Am stars increases with Vsini,
while it decreases for the overabundant elements. Chemical abundances of
various elements appear correlated with the iron abundance.Comment: Accepted for publication on A&
Spectroscopy of Blue Stragglers and Turnoff Stars in M67 (NGC 2682)
We have analyzed high-resolution spectra of relatively cool blue stragglers
and main sequence turnoff stars in the old open cluster M67 (NGC 2682). We
attempt to identify blue stragglers whose spectra are least contaminated by
binary effects (contamination by a binary companion or absorption by
circumstellar material). These ``best'' stragglers have metallicities ([Fe/H] =
-0.05) and abundance ratios of the blue stragglers are not significantly
different from those of the turnoff stars. Based on arguments from
hydrodynamical models of stellar collisions, we assert that the current upper
limits for the lithium abundances of all blue stragglers observed in M67 (by us
and others) are consistent with no mixing during the formation process,
assuming pre-main sequence and main sequence depletion patterns observed for
M67 main sequence stars. We discuss composition signatures that could more
definitively distinguish between blue straggler formation mechanisms in open
cluster stars.
We confirm the spectroscopic detection of a binary companion to the straggler
S 1082. From our spectra, we measure a projected rotational speed of 90+/-20
km/sec for the secondary, and find that its radial velocity varies with a
peak-to-peak amplitude of ~ 25 km/sec. Because the radial velocities do not
vary with a period corresponding to the partial eclipses in the system, we
believe this system is currently undergoing mass transfer. In addition we
present evidence that S 984 is a true blue straggler (and not an unresolved
pair). If this can be proven, our detection of lithium may indicate a
collisional origin.Comment: 20 pages, 4 figures, to appear in October 2000 A
Late stages of the evolution of A-type stars on the main sequence: comparison between observed chemical abundances and diffusion models for 8 Am stars of the Praesepe cluster
Aims. We aim to provide observational constraints on diffusion models that
predict peculiar chemical abundances in the atmospheres of Am stars. We also
intend to check if chemical peculiarities and slow rotation can be explained by
the presence of a weak magnetic field.
Methods. We have obtained high resolution, high signal-to-noise ratio spectra
of eight previously-classified Am stars, two normal A-type stars and one Blue
Straggler, considered to be members of the Praesepe cluster. For all of these
stars we have determined fundamental parameters and photospheric abundances for
a large number of chemical elements, with a higher precision than was ever
obtained before for this cluster. For seven of these stars we also obtained
spectra in circular polarization and applied the LSD technique to constrain the
longitudinal magnetic field.
Results. No magnetic field was detected in any of the analysed stars. HD
73666, a Blue Straggler previously considered as an Ap (Si) star, turns out to
have the abundances of a normal A-type star. Am classification is not confirmed
for HD 72942. For HD 73709 we have also calculated synthetic Delta-a photometry
that is in good agreement with the observations. There is a generally good
agreement between abundance predictions of diffusion models and values that we
have obtained for the remaining Am stars. However, the observed Na and S
abundances deviate from the predictions by 0.6 dex and >0.25 dex respectively.
Li appears to be overabundant in three stars of our sample.Comment: Accepted for publication on A&
Blue Stragglers in Galactic Open Clusters and the Integrated Spectral Energy Distributions
Synthetic integrated spectral properties of the old Galactic open clusters
are studies in this work, where twenty-seven Galactic open clusters of ages >=
1Gyr are selected as the working sample. Based on the photometric observations
of these open clusters, synthetic integrated spectrum has been made for the
stellar population of each cluster. The effects of blue straggler stars (BSSs)
on the conventional simple stellar population (SSP) model are analyzed on an
individual cluster base. It is shown that the BSSs, whose holding positions in
the color-magnitude diagrams (CMDs) cannot be predicted by the current
single-star evolution theory, present significant modifications to the
integrated properties of theoretical SSP model. The synthesized integrated
spectral energy distributions (ISEDs) of our sample clusters are dramatically
different from the SSPs based on isochrone only. The BSSs corrected ISEDs of
stellar populations show systematic enhancements towards shorter wavelength in
the spectra. When measured with wide-band colors in unresolvable conditions,
the age of a stellar population can be seriously under-estimated by the
conventional SSP model. Therefore, considering the common existence of BSS
component in real stellar populations, a considerable amount of alternations on
the conventional ISEDs should be expected when applying the technique of
evolutionary population synthesis (EPS) to more complicated stellar systems.Comment: 45 pages, 21 figures Accepted for publication in ApJ (Feburary 1,
2005 issue
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